Fatty acids have been used as raw materials in various applications in the chemical industry, typically in the manufacture of products ranging from lubricants, polymers, fuels and solvents to cosmetics. Fatty acids are generally obtained from wood pulping processes or by hydrolysis of triglycerides of vegetable or animal origin. Naturally occurring triglycerides are usually esters of glycerol and straight chain, even numbered carboxylic acids having 10-26 carbon atoms. Most common fatty acids contain 16, 18, 20 or 22 carbon atoms. Fatty acids may either be saturated or they may contain one or more unsaturated bonds. Unsaturated fatty acids are often olefinic having carbon-carbon double bonds with cis configuration. The unsaturated centres appear in preferred positions in the carbon chain. The most common position is ω9, like in oleic acic (C18:1) and erucic acid (C22:1). Polyunsaturated acids generally have a methylene-interrupted arrangement of cis-olefinic double bonds.
Saturated long straight chain fatty acids (C10:0 and higher) are solid at room temperature, which makes their processing and use difficult in a number of applications. Unsaturated long chain fatty acids like e.g. oleic acid are easily processable liquids at room temperature, but they are unstable because of double bond(s).
Branched fatty acids mimic the properties of straight chain unsaturated fatty acids in many respects, but they are more stable. For example branched C18:0 fatty acid, known as isostearic acid, is liquid at room temperature, but it is not as unstable as C18:1 acid, since the unsaturated bonds are absent in branched C18:0. Therefore, branched fatty acids are more desirable for many applications than straight chain fatty acids.
Diesel fuels based on biological material are generally referred to as biodiesel. A definition for “biodiesel” is provided in Original Equipment Manufacturer (OEM) guidelines as follows: Biodiesel is mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats, which conform to ASTM D6751 or EN 14214 specification for use in diesel engines as described in following Table 1. Biodiesel refers to pure fuel before blending with diesel fuel (B100).
TABLE 1Specification for Biodiesel (B100, 100%)PropertyASTM D6751EN 14214UnitsDensity at 15° C.860-900kg/m3Flash point (closed cup)130≧120° C.Water and sediment≦0.050≦0.050%Kinematic viscosity 40° C.1.9-6.03.5-5.0mm2/sSulfated ash≦0.020≦0.020% massSulfur≦0.05≦0.001% massCetane number≧47≧51Carbon residue≦0.050% massCarbon residue 10% dist≦0.3% massbottomAcid number≦0.80≦0.5mg KOH/gFree glycerol≦0.020≦0.02% massTotal glycerol≦0.240≦0.25% massPhosphorus content≦0.001≦0.001% mass
High cetane number, proper viscosity range and good low-temperature properties are required for a good diesel fuel. Cetane number (CN) has been established for describing the ignition quality of diesel fuel or its components. Branching and chain length influence CN, the CN decreasing with decreasing chain length and increasing branching. Hexadecane C16H34 has a CN of 100, and 2,2,4,4,6,8,8-heptamethylnonane C16H34 has a CN of 15. From structural features also double bonds decrease CN. Further, compounds with unsaturation can cause gumming in engines.
Besides CN, gross heat of combustion (HG) of a compound is essential in providing the suitability of the compound to be used as diesel fuel. For comparison the HGs of paraffinic and ester biodiesels are as follows: HG of hexadecane is 2559 kg cal/mol at 20° C. and of methyl palmitate (C16:0) 2550 kg cal/mol.
Cloud point presents the temperature where a petroleum product shows just a cloud or haze of wax crystals when it is cooled under standard test conditions, as described in standard ASTM D2500. Cloud point measures the ability of the fuel to be used in cold weather without plugging filters and supply lines.
Pour point is the lowest temperature at which a fuel will just flow when tested under the conditions described in standard ASTM D97. It is recommended by engine manufacturers that the cloud point should be below the temperature of use and not more than 6° C. above the pour point. Branching, saturation and chain length influence also cloud and pour points and they decrease with decreasing chain length, increasing unsaturation and increasing branching.
The viscosity of vegetable oils is approximately one order of magnitude greater than that of conventional diesel fuels. High viscosity results in poor atomization in combustion chamber, thus causing coking of nozzles and deposits.
Biodiesel is an alternative fuel, produced from renewable sources and it contains no petroleum. It can be blended in minor amounts with petroleum diesel to create a biodiesel blend, further it is non-toxic and essentially free of sulfur and aromatics. It can be used in compression-ignition (diesel) engines with little or no modifications. Diesel fuels based on biological material have been demonstrated to have significant environmental benefits in terms of decreased global warming impacts, reduced emissions, greater energy independence and a positive impact on agriculture.
It has been demonstrated that the use of diesel fuels based on biological material will result in a significant reduction in carbon dioxide emissions. A biodiesel lifecycle study of 1998, jointly sponsored by the US Department of Energy and the US Department of Agriculture, concluded that biodiesel reduces net CO2 emissions by 78 percent compared to petroleum diesel. This is due to biodiesel's closed carbon cycle. CO2, released into the atmosphere when burning biodiesel, is recycled by growing plants, which are later processed into fuel. As such, the increased use of diesel fuels based on biological material represents an important step to meet the emission reduction target as agreed under the Kyoto agreement. It is also believed that particulate emissions and other harmful emissions, such as nitrogen oxides, alleviating human health problems, are reduced.
Methyl esters of long-chain acids have higher cloud and pour points than the corresponding triglycerides and conventional diesel fuels. Cloud and pour points are important features when operating engines in cooler environment.
Several approaches, as such transesterification, dilution, micro-emulsification and co-solvent blending, as well as pyrolysis have been suggested for obtaining diesel fuel from vegetable oils and other triacylglycerol based feedstocks. The object of said approaches is to reduce the high kinematic viscosity of neat vegetable oils, which can cause severe operational problems and improper atomization of the fuel.
In transesterification, triglycerides forming the main component in vegetable oils are converted into the corresponding esters with an alcohol in the presence of catalysts. Methanol is the most commonly used alcohol due to its low cost and easy separation from the resulting methyl ester and glycerol phases.
Diluting 0-34% of vegetable oils with conventional diesel fuel leads to proper atomization but causes engine problems similar to those with neat vegetable oils.
Micro-emulsion fuels are composed of conventional diesel fuel and/or vegetable oil, a simple alcohol, an amphiphilic compound such as a surfactant and a cetane improver. Trace quantities of water are usually required for formation of the microemulsion.
Pyrolytic methods, Kolbe electrolysis and thermal and catalytic cracking of bio-materials like vegetable oils, their methyl esters and animal fats result in a wide spectrum of products, such as alkanes, alkenes, aromatics and carboxylic acids. The reactions are generally unselective and less valuable by-products are formed too.
Unsaturated and aromatic hydrocarbons present in the liquid fraction make the products obtained by the above methods unattractive for the diesel pool. Poor low-temperature properties of the products limit their wider use as biodiesel in regions with colder climatic conditions. In addition, the presence of oxygen in esters results in undesirable higher nitrogen oxide (NOx) emissions compared to conventional diesel fuels.
Sulphur free fuels are required in order to gain the full effect of new and efficient anti-pollution technologies in modern vehicles and to cut emissions of nitrogen oxides, volatile hydrocarbons and particles, as well as to achieve direct reduction of sulphur dioxide in exhaust gases. The European Union has decreed that these products must be available to the market from 2005 and must be the only form on sale from 2009. This new requirement will reduce annual sulphur emissions from automotive fuels.
Branched fatty acids and fatty acid esters, mainly methyl and ethyl esters, are obtained by isomerisation of straight chain, unsaturated fatty acids and fatty acid esters having a corresponding chain length, as described in U.S. Pat. No. 5,856,539. For example, branched C18:0 acids are prepared from straight chain C18:1 acids or also C18:2 acids.
Decarboxylation of carboxylic acids to hydrocarbons by contacting carboxylic acids with heterogeneous catalysts was suggested by Maier, W. F. et al: Chemische Berichte (1982), 115(2), 808-12. Ni/Al2O3 and Pd/SiO2 catalysts were tested for decarboxylation of several carboxylic acids. During the reaction the vapours of the reactant passed through a catalytic bed together with hydrogen at 180° C. and 0.1 MPa. Hexane represented the main product of the decarboxylation of heptanoic acid. When nitrogen was used instead of hydrogen no decarboxylation was observed.
U.S. Pat. No. 4,554,397 discloses a process for the manufacture of linear olefins from saturated fatty acids or esters by decarboxylation using a catalytic system, which consists of nickel and at least one metal selected from the group consisting of lead, tin and germanium. Additives may be included in the above-mentioned catalysts and for example sulphur derivatives may be added to decrease the hydrogenating power of nickel and make the reaction more selective for olefin formation reaction. The presence of hydrogen was necessary to maintain the activity of the catalyst. The reaction was carried out at a temperature of 300-380° C. and the pressure was atmospheric pressure or higher.
Decarboxylation accompanied with hydrogenation of oxo-compound is described in Laurent, E., Delmon, B.: Applied Catalysis, A: General (1994), 109(1), 77-96 and 97-115, wherein hydrodeoxygenation of biomass derived pyrolysis oils over sulphided CoMo/Al2O3 and NiMo/Al2O3 catalysts was studied. Hydrotreating conditions were 260-300° C. and 7 MPa in hydrogen. The presence of hydrogen sulphide promoted the decarboxylation, particularly when a NiMo catalyst was used.
Unsaturated and aromatic hydrocarbons produced in the side reactions in the above-mentioned processes make the obtained products unattractive for the diesel pool. In addition, the unbranched and highly saturated structures cause poor low-temperature properties.
FI 100248 describes a two-step process for producing middle distillate from vegetable oil by hydrogenating fatty acids or triglycerides of vegetable oil using commercial sulphur removal catalysts (NiMo and CoMo) to give n-paraffins and then by isomerising said n-paraffins using metal containing molecule sieves or zeolites to obtain branched-chain paraffins. The hydrotreating was carried out at reaction temperatures of 330-450° C.
Based on the above it can be seen that here is a need for a new alternative process for the preparation of saturated and branched hydrocarbons from renewable sources, suitable as biodiesel of high quality.